1. Field of the Invention
This invention relates to a recording medium having a texture like that of paper made of pulp fibers, a good ink absorbing effect and physical properties that make the recording medium good for being written on with a pencil. Such a recording medium can suitably be used for color recording using aqueous ink and particularly for color recording using an ink-jet recording method. The present invention also relates to a method of manufacturing such a recording medium and an image forming method for forming images by using such a recording medium.
2. Related Background Art
Ink-jet recording systems are adapted to record images and/or characters on a recording medium (such as paper) by ejecting fine droplets of ink based on any of the currently known various operation principles. An ink-jet recording system provides advantages including high speed printing, a low noise emission level, adaptability to multi-color printing, versatile recording pattern forming capabilities and elimination of development and fixing processes and therefore has been finding an increasingly large number of applications. Furthermore, currently known multi-color ink-jet recording systems can form multi-color images with an image quality comparable to that of color images printed by using a conventional printing system of preparation type or a color photography system at cost-lower than ordinary color printing if the number of copies is relatively small. Therefore, they are being used also in the field of full color image recording.
While improvements have been made to ink-jet recording apparatus and ink-jet recording methods to meet the demand for a higher recording speed, a higher image definition and a full color printing, such a rigorous demand has also been directed to the recording medium to be used for ink-jet recording. For the ink-jet recording system, ink containing an aqueous solvent that may be water or a mixed solution of water and an organic solvent to a large extent is normally used because ink droplets have to be ejected at high speed from nozzles toward the recording medium. For recording color images with a high color density, it is therefore necessary to use ink at a high consumption rate. On the other hand, the beading phenomenon of combined and fused ink dots can appear to disturb the image printing operation because ink droplets are ejected continuously. In order to prevent the beading phenomenon, the recording medium to be used with the ink-jet recording system is required to absorb ink at a high rate and to a large extent.
Various forms of recording mediums have been proposed to meet the above-listed requirements. For instance, there are two known types of ink-jet recording sheets including the ordinary paper type such as wood-free paper and bond paper and the coated type having an ink receiving layer (film coat) formed on a support member (to be referred to as base paper or base member hereinafter) that may be a sheet of paper such as wood-free paper, synthetic paper or synthetic resin film. The coated type may be subdivided into the low coating rate type with a coating rate between 1 and 10 g/m2, the medium coating rate type with a coating rate between 10 and 20 g/m2 and the high coating rate type with a coating rate higher than 20 g/m2.
While ink-jet recording sheets with a coating rate equal to or higher than that of the medium coating rate type are adapted to produce fine and sharp images by ink-jet recording, the texture and other physical properties of the base paper are lost to some extent due to the thick coat layer. Additionally, they do not have physical properties that make them good for being written on with a pencil. For these reasons, there is a demand for a recording medium of the low coating rate type that has an agreeable texture and other physical properties including being good for being written on in pencil, and is still adapted to produce fine and sharp images.
In the case of a recording medium of the low coating rate type, it is difficult for the ink receiving layer to absorb all the ink applied to it and the base paper has to be made responsible for part of the applied ink. For example, Japanese Patent Publication No. 3-26665 and Japanese Patent Applications Laid-Open Nos. 59-38087 and 59-95186 describe the use of base paper showing a low Stöckgt sizing degree. When a base member showing a low Stöckgt sizing degree is used, no spills and blurs of ink nor the so-called beading phenomenon of producing agglomeration of ink and resultant uneven printing occur on the surface because the high ink absorbability of the base member is exploited.
On the other hand, however, with any known recording medium of the low coating rate type, ink can penetrate deep into the inside of the base member to make it impossible to raise the density of the recorded image. Since the known recording medium of the low coating rate type uses base paper showing a relatively low sizing degree and the ink absorbability of the recording medium mainly relies on the base paper itself, the fiber coating ratio of the ink receiving layer is insufficient and pulp fibers can be remarkably exposed on the surface of the recording medium if the coating rate is low and the coating formulation cannot be applied uniformly to undulated areas where pulp fibers of the base paper are intertwined. When an image is formed on such a recording medium particularly by using aqueous ink, ink can be dispersed along coarse pulp fibers to give rise to a phenomenon of feathering from the periphery of printed dots to make it impossible to produce really circular dots.
Japanese Patent Publication No. 63-22997 described a technique of forming an ink receiving layer on the surface of a support member, making the pore size distribution curve of the uppermost layer show a peak between 0.2 μm and 10 μm and regulating the peaks of the entire ink receiving layer so as to be well balanced. With this technique, pores having a large pore radius between 0.2 μm and 10 μm are formed in the uppermost layer without fail. Then, a high absorptive power (ink absorption rate) is secured mainly by the double layer structure. However, the above technique cannot make the base sheet fully exhibit its physical properties including the texture and the color tone. Additionally, since it does not exploit the gap structure of the base paper, a satisfactory ink absorbing effect cannot be achieved depending on the Stöckigt sizing degree and the surface profile of the base paper.